There's probably a much greater amount of energy lost from conversion to liquid form and stored and converted back into usable electricity from steam generators. I believe the article mentions it but it likely has better long term storage than batteries as well. The equipment required for the 'air' battery would have a longer lifespan and be cheaper to repair then a massive battery bank. On top of that batteries aren't exactly the most 'green' thing to produce, so from a carbon footprint perspective, I imagine the air battery is much more efficient.